Traditional telephony carriers (Public Switched Telephony networks or PSTN) are moving towards a packet-based VoIP infrastructure. A key component of any telephony infrastructure is Call Control. Call control includes call setup and teardown. In VoIP, call control may be achieved through SIP. SIP is a standard protocol for VoIP messages. The SIP messages are text-based.
A key requirement for a robust VoIP call control infrastructure is overload control. Since VoIP is an emerging area and different components of the infrastructure are presently in a trial phase, large scale deployments of VoIP has not yet taken place. Once large scale deployments begin, call control in VoIP infrastructure will become critical in avoiding service outages and loss of revenue. Since VoIP is an emerging area, there has been little work on overload protection of VoIP servers.
In addition to landline telephony services, SIP has also been chosen as a signaling mechanism for third generation (3G) mobile telephony services, specifically by the 3GPP standards body in its release UMTS R00. The implication of this development is that overload protection for the SIP-based call control infrastructure is needed both for landline and mobile telephony services. A second implication is that in mobile telephony services, the control infrastructure has to support not only call setup and teardown, but the control infrastructure also has to handle terminal mobility between different cell boundaries. This support for hand-off represents an additional load on the call control servers.
Another important development is the use of SIP to carry instant messages (IM) and for supporting Presence. Presence is a term understood by persons of ordinary skill in the art and refers to the concept of letting interested users in the system know who is online/offline. For example, a SameTime server shows a “green” light next to someone logged in. Two methods are currently under standardization at the IETF standards body. One method, called SIMPLE, carries Instant Messaging payloads in SIP messages that conceivably will be processed by the same call control infrastructure, as networks moved towards offering converged voice and data services. Using SIP for Instant Messaging and Presence implies an additional load on the SIP based call control infrastructure.
In addition to handling call setup, teardown, hand-off and Instant Messaging, SIP call control servers need to also exchange routing information with other call control servers in order to route call setup messages appropriately.
For the reasons set forth above, there is a need in the art for a method and system of call control for SIP messages for VoIP.